Due to its recognized, though insufficiently defined, graft-versus-tumor (GVT) activity, allogeneic hematopoietic stem cell transplantation (alloHSCT) is currently the only curative treatment available for patients with certain hematologic malignancies including B cell chronic lymphocytic leukemia (B-CLL). GVT activity, as well as its counterpart graft-versus-host-disease (GVHD), is believed to be mediated primarily by alloreactive donor T cells. Shifting the focus to another component of the adaptive immune system, we are investigating whether alloreactive antibodies derived from donor B cells may also have a role in GVT activity. Specifically, we are interested in alloHSCT-induced antibodies that selectively recognize tumor cell surface antigens and are capable of mediating antibody-dependent cellular cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC). In addition to their possible involvement in GVT activity, these antibodies may serve as tools for the discovery of new targets for human monoclonal antibody therapy. We developed a sensitive flow cytometry assay for detection of post-alloHSCT serum antibodies against cell surface antigens. Post-alloHSCT sera collected from B-CLL patients at defined time points after transplantation showed significant binding to B-CLL tumor cells but not normal B cells. Pre-alloHSCT sera, donor sera, and control sera were negative. The alloreactive antibody response correlated with the disappearance of circulating B-CLL tumor cells, suggesting an antigen-driven immune response. In order to identify the alloreactive antibodies and subsequently the cell surface antigens they recognize, we generated a human antibody library from post-alloHSCT peripheral blood mononuclear cells (PBMC) and selected it on primary B-CLL tumor cells by phage display. We are currently screening the selected antibodies for B-CLL cell surface binding. In summary, we have shown that (i) alloHSCT induces an antibody response to tumor cell surface antigens and (ii) this antibody response may be harnessed for human monoclonal antibody drug and target discovery. The identification and characterization of B-CLL-specific antigens could open avenues for the development of more effective and precise immunotherapeutic strategies. Recent gene expression profiling in B-CLL identified several genes that are uniquely expressed by B-CLL cells, although the expression and function of the corresponding proteins, and more importantly their suitability as candidate target antigens remain largely unknown. We have analyzed the cell surface expression of one such protein, receptor tyrosine kinase-like orphan receptor 1 (ROR1), on B-CLL cells. Collectively, our ROR1 protein expression analysis in human B-CLL suggests that, based on its highly restricted, consistent, and constitutive expression, ROR1 protein may be a promising target antigen for monoclonal antibody therapy of B-CLL.